Optical submarine cables are used for transmitting optical signals. Such cables typically have a plurality of optical fiber pairs which are used for transmission of optical signals. In certain applications it may be required that different fiber pairs are allocated to different customers, such that the entire capacity in one fiber pair is allocated to one particular customer. In such a case, there is no possible access by other customers that are allocated other fiber pairs, to the fiber pair of that particular customer. However, in certain cases, the capacity inside one fiber pair may be shared between various customers. Typically, the fiber may carry various wavelengths, and each wavelength is intended for use of a separate customer. In such cases a wavelength add-drop branching unit is inserted in the system and thereby some wavelengths are routed towards a branch station. In submarine networks, optical cables may be laid underwater for very long distances such that the cable may traverse waters neighboring various countries or even continents. Branching units then serve for extracting (dropping) from the main transmission line one or more wavelengths and direct them to a terminal station at the end of the branch; and/or incorporating (adding) one or more wavelengths coming from the terminal station into the main transmission line. Furthermore, in order to compensate for loss in the signal power during transmission, especially over long distances, repeaters are used at suitable locations along the main transmission line or along the branch lines. These repeaters typically allow the transmission of large bandwidths (typically higher than 18 nm) which incorporate a plurality of wavelengths.